Landlords are taking their flats off the open market, and advertising them instead on the holiday let website Airbnb, warns an industry report.
The Residential Landlords Association (RLA) claims the trend will mean fewer homes for rent, exacerbating the housing shortage.
It claims that rents could rise as a result, particularly in London.
And it says 61% of properties advertised on Airbnb in the capital are available for more than three months.
As a result, some landlords may be deliberately avoiding regulations that apply to long-term lets in the private rented sector.
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The RLA is calling on the government and the Mayor of London to police websites like Airbnb, to make sure that landlords are not using them to get around the law.
“Given the pressures faced in the capital it is important that properties advertised as being available for more than 90 days a year are genuine holiday lets with appropriate planning permission,” said RLA policy director David Smith.
“Otherwise, as well as taking rental stock off the market for those looking for somewhere to live, they are also putting tenants in a vulnerable position without all the protections offered by a tenancy agreement.”
The RLA is also concerned that many tenants may be sub-letting flats to short-term visitors.